1. Field of the Invention
The present invention relates to antiglare film, method of producing the same, anti-reflection film, polarizing plate, and image display device. More particularly, the present invention relates to antiglare film, and a method of producing the same both with high performance and with great ease, and anti-reflection film, polarizing plate, and image display device.
2. Description Related to the Prior Art
Anti-reflection film is provided in several sorts of image display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electro luminescence display (ELD), a cathode-ray tube (CRT) and the like. The anti-reflection film is used for an eyeglass, of a lens incorporated in a camera. Several types of the anti-reflection films have been proposed. Some of them have a multi-layers structure or a nonuniform layer structure, and are widely used. A film support is provided with a plurality of transparent layers of metal oxides, so as to prevent the reflection in a wide wavelength range of a visible ray.
Such transparent layers of metal oxides are usually formed in methods of vapor deposition. As the methods, there are chemical vapor deposition (CVD) and physical vapor deposition (PVD). Specifically in the PVD, some substances are evaporated, such that a gas thereof in form of molecules or atoms forms a thin layer. The PVD is often made in vacuum deposition method and sputtering method. However, those are unsuitable for mass production, due to low productivity.
In production of the anti-reflection film, the PVD is often carried out on a film support, while a metal layer on the film support is provided with the protruding and retreating surface pattern in accordance with the way of use. In this type of the anti-reflection film, parallel transmittance becomes lower than in the anti-reflection film having a smooth surface on which that vapor deposition is performed. As the protruding and retreating surface pattern scatters the external light to suppress mirroring. The produced anti-reflection film has antiglare property. Accordingly, such anti-reflection films improve the display quality of the image display device.
Instead of the methods of vapor deposition, the following publications propose methods of producing the anti-reflection film by coating a film support with a solution containing inorganic micro particles for forming an anti-reflection layer: JP-B 60-059250 (corresponding to JP-A 56-084729) and JP-A 59-050401. In JP-B 60-059250, a solution is cast on a film support to form an anti-reflection layer including inorganic micro particles and micro voids. After the solution is dried and forms an anti-reflection layer on the film support, it is processed in gas activation. Thereby, a gas leaves the coating layer, and the micro voids are formed in the coating layer. JP-A 59-050401 discloses a multi-layer structure having a support, a high refractive index layer and a low refractive index layer overlaid on the former and formed from coating of polymer or inorganic micro particles. In addition, the document suggests provision of a middle refractive index layer disposed between the support and the high refractive index layer.
JP-A 2-245702 discloses anti-reflection film in which micro particles of two or more compounds, such as MgF2 and SiO2, are contained, and a proportion of mixture of those is changed in the thickness direction of the film. According to JP-A 2-245702, the change in the proportion of the compounds for the particles is effective in changing the refraction index within the film, to obtain a similar optical effect to that of JP-A 59-050401 disclosing the high and low refractive index layers in the dual layer structure. In this anti-reflection film, the micro particles are fixed to a support through SiO2 produced in thermal decomposition of the ethyl silicate. In the thermal decomposition, carbon dioxide and gaseous water are generated from the low refractive index layer through the combustion of the ethyl group. Thereby, micro voids are formed between the micro particles in the low refractive index layer.
JP-A 5-013021 teaches the improvement of the anti-reflection film of JP-A 2-245702. In the improvement, the micro voids are filled with binder. Further, JP-A 7-048527 teaches anti-reflection film containing binder and inorganic particles of porous silica. Furthermore, JP-A 11-006902 discloses a three-layer structure of film in which a low refractive index layer is overlaid on a support and includes plural inorganic micro particles together with micro voids, and coated with coatings in a wet manner. This is characterized in application of all the coatings in the wet manner at a reduced manufacturing cost, and has an intention of strengthening the film even with a lowered reflection.
To impart antiglare properties to the anti-reflection film, various methods are known, including application of a coating an anti-reflection layer to an initially roughened support with a protruding and retreating pattern. Another method is to add mat particles to coating solution for an anti-reflection layer in order to form a protruding retreating surface pattern. JP-A 2000-275401 and 2000-275404 propose improvements of the anti-reflection films in JP-A 11-006902. At first, a flat anti-reflection film is produced, and a surface thereof is embossed to form the protruding and retreating surface pattern.
In general, the antiglare film or anti-reflection film is secured and positioned on the outermost side of an image display device. Typically when the film is used in a display device for a television set, computer, portable digital electronic instrument or the like, incidental outer force is likely to be exerted to the film. So high resistance to force to scratch, depress or damage the film is required in the practical use. To keep resistance to outer force high, it is conceivable to raise smoothness of the surface by lowering friction or surface energy of the surface, or to raise force of bonds between the plural layers overlaid on one another for high resistance against being peeled. However, it is basically necessary to add at least one hard coat layer which has a characteristically high hardness sufficient for being resistant to outer force. The hard coat layer is directly adjacent to the support of polymer film, has a thickness from several microns to tens of microns, and is formed to have hardness which corresponds to pencil hardness of H or higher, preferably 2H or higher.
Although the plural documents disclose micro voids between micro particles in the anti-reflection film, no document discloses optical characteristic of the illustrated voids. In these anti-reflection films, the micro voids are filled with the binder such that the anti-reflection film may be stronger. However, when the micro voids are filled with the binder, it becomes harder to decrease the refractive index of the anti-reflection film enough.
Also, a serious problem arises in failure in reproduction in good quality due to forming of above-described protruding and retreating pattern for antiglare properties. If a partial protruding or retreating region on the surface is larger than a cell included in numerous image display cells, it is likely that transmitted light through a single cell is condensed by a lens effect of the partial region. Also, light components of red, green and blue transmitted light are unacceptably mixed in the color by the partial regions if passed through adjacent cells. Otherwise, not only the wide view angle and the high speed response but also the high definition are required so much to obtain a high image quality. The high definition is realized by decreasing a cell size. In this case, for example, when the cell size is so smaller that the display has at least 133 ppi (pixel per inch), then the light transmits through the anti-reflection film, and the light perceived by a user has the nonuniform brightness, which cases the dazzling on the display. Therefore, the quality of the anti-reflection film becomes lower as a product.
A problem in providing the hard coat layer lies in the insufficiency of the embossed protruding and retreating pattern both in the depth direction and in the two-dimensional direction. This is because the embossing for imparting the antiglare properties is partially blocked by the hardness of the hard coat layer. Unwanted results occur in insufficiency in the antiglare effects, or low performance according to the dazzling appearance.